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The role of yttrium doping on the structural and superconducting properties of SmFeAsO $$_{1-x}$$ F $$_x$$ (Sm1111) is studied in detail and is compared with other yttrium-doped RE1111 (RE-rare earth) iron oxypnictide superconductors on the basis of the existing reports. It is found that the partial substitution of lanthanides such as La, Ce, and Nd with yttrium systematically enhances $$T_\mathrm{C}$$ of the system to different levels depending on their crystal ionic radii difference with yttrium. The isovalent substitution of these lanthanides at relatively smaller yttrium ion site causes inner chemical pressure and enhances the $$T_\mathrm{C}$$ . However, yttrium doping does not seem to be beneficial for Sm1111 because of its larger ionic size. [ABSTRACT FROM AUTHOR]

The optical properties of the superconducting single crystal Rb0.8Fe1.68Se2 with Tc ... 31 K have been measured over a wide frequency range in the ab plane. We found that the optical conductivity is dominated by a series of infrared-active phonon modes at low-frequency region as well as several other high-frequency bound excitations. The low-frequency optical conductivity has rather low value and shows quite small Drude-like response, indicating low carriers density in this material. Furthermore, the phonon modes increase continuously in frequency with decreasing temperature; specifically, the phonon mode around 200 cm-1 shows an enhanced asymmetry effect at low temperatures, suggesting an increasing electron-phonon coupling in this system. [ABSTRACT FROM AUTHOR]

Abstract: The transport and magnetic properties of the undoped and yttrium doped NdFeAs(O,F) superconductor are investigated in detail for the first time. The transition temperatures of the yttrium doped samples are found to increase monotonically and attain a maximum of 53.3K for 30% yttrium doping, ∼4K higher as compared to the yttrium free sample. The TC values observed from the temperature dependent ac susceptibility measurements are also in agreement with the resistivity measurements. Yttrium doping is found to decrease the lattice parameters of the system; causing an inner chemical pressure and thereby accomplishing a TC enhancement. Moreover, the substitution of relatively smaller yttrium ion at Nd site creates lattice defects which act as effective pinning centers thereby enhancing the JC (H) performance of the system. [Copyright &y& Elsevier]

Abstract: In order to elucidate the details of successive magnetic transitions in the rare earth cobalt arsenic oxides (RECoAsO), we have measured muon-spin rotation and relaxation(μ +SR) spectra of powder samples with RE = La, Ce, Pr, Nd, Sm, and Gd. Together with the result of magnetization measurements, it was found that all the compounds enter into abulk static ferromagnetic ordered phase below around 70 K(=T C). Furthermore, additional transitions into a static antiferromagnetic ordered phase were found for the RE = Nd, Sm, and Gd compounds; namely T N1 = 15 K and T N2 = 7 K for Nd, T N1 = 41.5 K and T N2 = 17.5 K for Sm, and T N = 3K for Gd. [Copyright &y& Elsevier]

In this paper we analyze transport data of different families of Fe-based superconductors, within a pseudogap framework. The Fe(Te, Se) samples exhibit s-shaped resistivity ρ( T) curves, while SmFeAs(O, F) ones present a departure from their high-temperature linear behavior. In either case, a characteristic temperature can be identified. These temperatures correspond to those at which abrupt changes in the temperature behavior of Hall resistance and of Seebeck coefficient occur, suggesting that they are signatures of pseudogaps opening in the density of states. The direct correlation between these characteristic temperatures and the superconducting transition temperatures suggests that the pseudogap and the superconducting state originate from the same mechanism. Scaling procedures of resistivity curves confirm such proportionality. On the other hand, excess Fe content in Fe(Te, Se) samples affects the pseudogap temperature much more strongly than the superconducting T. Finally, we find out that the pseudogap in the 1111 family is almost insensitive to disorder, as in high- T cuprates. [ABSTRACT FROM AUTHOR]

Abstract: Isoelectronic Tellurium (Te) substitution for Selenium (Se) in the tetragonal phase of FeSe (β-FeSe) increases the superconducting transition temperature (Tc ) by applying a negative pressure on the lattice. However, the normal state resistivity increases and shows semi-metallic behavior for samples with higher Te concentration. With increasing Te concentration, the Tc increases and reaches a maximum for FeSe0.5Te0.5 and then decreases with further increase of Te. We have investigated the effect of Cobalt (Co) and Nickel (Ni) doping in FeSe0.5Te0.5 in the nominal composition range Fe1− x TM x Se0.5Te0.5 (TM=Co (x =0.05, 0.1, 0.15, 0.2) and Ni (x =0.05, 0.1)). Both Co and Ni doping suppress Tc and drives the system to metal–insulator transition. The in-plane (‘a’) and out-of-plane (‘c’) lattice constants decrease with increasing dopant concentration. [Copyright &y& Elsevier]

A carrier doping by a hydrogen substitution in LaFeAsO 1- x H x is known to cause two superconducting (SC) domes with the magnetic order at both end sides of the doping. In contrast, SmFeAsO 1- x H x has a similar phase diagram but shows single SC dome. Here, we investigated the electronic and crystal structures for iron oxynitride Ln FeAsO 1- x H x ( Ln = La, Sm) with the range of x = 0-0.5 by using x-ray absorption spectroscopy, x-ray emission spectroscopy, and x-ray diffraction. For both compounds, we observed that the pre-edge peaks of x-ray absorption spectra near the Fe- K edge were reduced in intensity on doping. The character arises from the weaker As-Fe hybridization with the longer As-Fe distance in the higher doped region. We can reproduce the spectra near the Fe- K edge according to the Anderson impurity model with realistic valence structures using the local-density approximation (LDA) plus dynamical mean-field theory (DMFT). For Ln = Sm, the integrated-absolute difference (IAD) analysis from x-ray Fe- Kβ emission spectra increases significantly. This is attributed to the enhancement of magnetic moment of Fe 3 d electrons stemming from the localized picture in the higher doped region. A theoretical simulation implementing the self-consistent vertex-correction method reveals that the single dome superconducting phase for Ln = Sm arises from a better nesting condition in comparison with Ln = La., (© 2021 IOP Publishing Ltd.)

We examine electronic and crystal structures of iron-based superconductors Ln FeAsO 1- x H x ( Ln = La, Sm) under pressure by means of x-ray absorption spectroscopy (XAS), x-ray emission spectroscopy (XES), and x-ray diffraction. In LaFeAsO the pre-edge peak on high-resolution XAS at the Fe- K absorption edge gains in intensity on the application of pressure up to 5.7 GPa and it saturates in the higher pressure region. We found integrated-absolute difference values on XES for Ln = La, corresponding to a spin state, decline on the application of pressure, and then it is minimized when the T c approaches the maximum at around 5 GPa. In contrast, such the optimum value was not detected for Ln = Sm. We reveal that the superconductivity is closely related to the lower spin state for Ln = La unlike Sm case. We observed that As height from the Fe basal plane and As-Fe-As angle on the FeAs 4 tetrahedron for Ln = La deviate from the optimum values of the regular tetrahedron in superconducting (SC) phase, which has been widely accepted structural guide to SC thus far. In contrast, the structural parameters were held near the optimum values up to ∼15 GPa for Ln = Sm., (© 2021 IOP Publishing Ltd.)